Best practice: Runtime filters with Room and LiveData

Question

I am working on a screen that shows the contents of a Room wrapped DB using a recycler. The adapter gets the LiveData from a ViewModel that hides the query call on the Room DAO

Answer 1

I'm working in a similar problem. Initially I had RxJava but now I'm converting it to LiveData.

This is how I'm doing inside my ViewModel:

// Inside ViewModel
MutableLiveData modelFilter = new MutableLiveData<>();
LiveData> modelLiveData;

This modelLivedata is constructed in the following way inside view model constructor:

        // In ViewModel constructor
        modelLiveData = Transformations.switchMap(modelFilter,
                    new android.arch.core.util.Function>>() {
                        @Override
                        public LiveData> apply(FilterState filterState) {
                            return modelRepository.getModelLiveData(getQueryFromFilter(filterState));
                        }
                    });

When the view model receives another filter to be applied, it does:

// In ViewModel. This method receives the filtering data and sets the modelFilter 
// mutablelivedata with this new filter. This will be "transformed" in new modelLiveData value.
public void filterModel(FilterState filterState) {

    modelFilter.postValue(filterState);
}

Then, this new filter will be "transformed" in a new livedata value which will be sent to the observer (a fragment).

The fragment gets the livedata to observe through a call in the view model:

// In ViewModel
public LiveData> getModelLiveData() {

    return modelLiveData;

}

And inside my fragment I have:

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    ViewModel viewModel = ViewModelProviders.of(this.getActivity()).get(ViewModel.class);

    viewModel.getModelLiveData().observe(this.getViewLifecycleOwner(), new Observer>() {
        @Override
        public void onChanged(@Nullable PagedList model) {
            modelListViewAdapter.submitList(model);
        }
    });

}

I hope it helps.